How do we know if summers are getting hotter?

Think of a hot summer afternoon, when the sun warms your arms and your ice lolly drips down your fingers. Maybe you have heard a grown-up say that summers felt cooler when they were small.

But a feeling is not the same as knowing. Memories can play tricks, and one sticky day can feel like a whole hot summer. So how do we know if summers are really getting hotter?

Weather scientists start at a weather station, which means a special spot where a thermometer measures the air. The thermometer sits inside a white box that keeps it in the shade, with thin slits that let the air flow through. That way the sun cannot shine on it straight, and the number shows only how warm the air is.

The thermometer checks the air all day long, and the weather scientists keep the hottest number from every single day. They save these numbers year after year, so each weather station holds a long, long list. Some of those lists go back more than one hundred years.

Yet one hot day on its own tells us very little. A cool, rainy week can come right after a scorching one, because weather jumps up and down. So the scientists look at a whole summer of days together.

For each summer, they work out the average, which means the number you get when you share all the days' warmth out evenly. Imagine three days that reach twenty, twenty-two and twenty-four degrees. Share them out evenly, and the average is twenty-two, right in the middle. Now one number tells the story of a whole summer.

Let's picture one town and its weather station. The scientists put the averages from summers when your grandma was a child next to the averages from summers today. They line them up, year by year, like beads on a string. What do you think they see when they compare them?

The summer averages from today come out higher than the ones from long ago. In that town, today's summers are about one degree warmer than in grandma's day. One degree sounds tiny, but a whole summer's average only climbs when day after day of that summer runs warmer.

Still, one town could be a special case, so the scientists check other weather stations too. They look at thousands of them, in hot lands and cold lands, by the sea and up in the mountains. Almost everywhere, the summer averages climb too, and that is how we know summers on Earth are getting hotter.

Weather scientists keep the hottest number from every summer day at weather stations all over the world, and they work out each summer's average. When they compare those averages, today's summers come out higher than summers long ago, so we know summers are getting hotter.

Out in towns, up on mountains and beside the sea, little white boxes stand quietly through every summer. Inside each one, a thermometer measures the warm air, and the weather scientists add one more number to that station's long list.